1. Field of the Invention
This invention generally relates to systems and methods for inspection of surfaces of specimens such as semiconductor wafers. Certain embodiments relate to systems and method for contact image sensor based detection of defects on such surfaces.
2. Description of the Related Art
Fabrication of semiconductor devices such as logic and memory devices includes using a number of processes to form various features and multiple levels or layers that comprise semiconductor devices on the surface of a semiconductor wafer, or similar substrate. For example, lithography is a semiconductor fabrication process that typically involves transferring a pattern to a resist on the surface of a semiconductor wafer. Additional examples of semiconductor fabrication processes may include chemicalmechanical polishing, etch, deposition, and ion implantation. Semiconductor devices are far smaller than the substrates, or wafers, and an array of multiple identical semiconductor devices is formed on the wafer, and then separated into individual semiconductor devices after all processing is completed.
During each semiconductor fabrication process, defects such as particulate contamination and pattern defects may be introduced into the semiconductor devices. Such defects may be found either randomly on a wafer surface, or may be repeated within each device found on the wafer. For example, randomly placed defects may be caused by events such as an unexpected increase in particulate contamination in a manufacturing environment or an unexpected increase in contamination in process chemicals that are used in fabrication. Defects that are repeated in each semiconductor device appearing on the entire wafer may, for example, be systematically caused by contamination or defects found on the reticle, or mask, that may then be transferred along with the desired device pattern during the lithography process.
As the dimensions of advanced semiconductor devices continue to shrink, the presence of defects in the semiconductor device limit the successful fabrication, or yield, of a semiconductor device. For example, a reticle defect that is reproduced in a patterned resist during lithography may cause an open circuit or a short circuit in a semiconductor device formed in subsequent processing. Because fabrication of a semiconductor device is composed of many complex process steps, the effects of defects on the total yield typically increase exponentially if an error that is caused by a defect is propagated throughout an entire semiconductor device. Thus, identifying and eliminating the sources of defects at critical steps during the fabrication process is an important objective to minimize cost. In particular, detection of defects at the appropriate process step may make possible rework or correction of the wafer as well as correction of any abnormal process deviations.
Defects commonly found during the after develop step in lithography are typically “macro” in size, ranging from about ten micrometers to the hundreds of millimeter dimensions of the whole wafer. Typically macro-level defects are those having lateral dimension greater than about 25 μm, but some macro-level defects such as scratches may have one dimension less than 25 μm and another well over 25 μm. The discussion herein primarily refer to the application of the inventive apparatus and methods in the field of after-develop inspection (ADI), though the applications for the invention and its methods are not intended to be limited to the ADI application.
The types of such macro or large-scale defects are quite varied, even within the class of lithography-related process steps. For example, one kind of defect type includes those resulting from resist or developer problems such as lifting resist, thin resist, extra photoresist coverage, incomplete or missing resist which may be caused by clogged dispense nozzles or an incorrect process sequence, and developer or water spots. Other examples of defects include regions of defocus caused by particles on the back side of a wafer (“hot spots”), reticle errors such as tilted reticles, out-of-focus exposure or incorrectly selected reticles, scratches, pattern integrity problems such as over or under developing of the resist, contamination such as particles or fibers, and non-uniform or incomplete edge bead removal (“EBR”). The term “hot spot” generally refers to a photoresist exposure defect that may be caused, for example, by a depth of focus limitation of an exposure tool, an exposure tool malfunction, a non-planar surface of the semiconductor topography at the time of exposure, foreign material on a back side of the semiconductor topography or on a surface of a supporting device, or a design constraint. With the exception of non-uniform or incomplete EBR, such defects generally occur randomly or systematically from lot-to-lot or from wafer-to-wafer. As such, macro-level defect inspection may involve inspecting all of the wafers in a lot or only a number of wafers in each lot.
These macro-level defects found on specimen surfaces particularly after the development of resist patterns placed during the lithography process are typically monitored manually by visual inspection, because many of these macro-level defects generated during a lithography process may be visible to the naked eye. Defects that may be visible to the human eye typically have a lateral dimension greater than or equal to approximately 100 μm. Defects having a lateral dimension as small as 10 μm, however, may also be visible on unpatterned regions of a wafer surface, or semiconductor topography. Prior to the commercial availability of automated defect inspection systems such as the systems illustrated in U.S. Pat. No. 5,917,588 to Addiego and U.S. Pat. No. 6,020,957 to Rosengaus et al., which are incorporated by reference as if fully set forth herein, manual inspection using an un-aided eye was the most common, and may still be the most dominant, inspection method used by lithography engineers.
The simplest method of manually inspecting a specimen surface is to tilt a hand-held specimen under a bright light, and look for the macro-level defects by an un-aided eye. Methods that are semiautomatic, but still rely on such visual inspection where an un-aided eye is used, may involve, for example, placing the wafer specimen on a semiautomatic tilt table and rotating the wafer through various angles under a bright light. The semiautomatic tilt table may rotate the wafer about a central axis while positioning the wafer at different inclinations relative to a plane normal to the central axis. In this manner, an operator can then visually inspect (i.e. with the unaided eye) the wafer surface for defects as it rotates, and then qualitatively evaluate if the wafer is acceptable or not for further processing. An example of a visual inspection method is illustrated in U.S. Pat. No. 5,096,291 to Scott and is incorporated by reference as if fully set forth herein.
There are, however, several limitations to applying visual inspection methods, where the un-aided eye is used. Typically such visual inspection methods are time-consuming and may be subject to operator error. In addition, lithography and automation trends in the semiconductor industry are recognizing macro-level defect inspection as a critical step to maintaining or enhancing yield, and are thus seeking methods that are more repeatable and reliable than human inspectors. Thus, many automated inspection systems such as described in the prior art by Addiego are being adopted for defect inspection to decrease the time required to inspect specimen surfaces and to increase the accuracy of the inspection process.
Inspection systems such as those described by Addiego use light scattering techniques that are typically comprised of an illumination system and a detection system. The illumination system illuminates a surface of a specimen such as a wafer with a source of light such as a laser or broadband lamp. Any defects that are present on the surface will scatter the incident light. The detection system is configured to collect the scattered light which can be converted into electrical signals which can be measured, counted, and displayed on an oscilloscope or other monitor. Examples of such inspection systems are illustrated in U.S. Pat. No. 4,391,524 to Steigmeier et al., U.S. Pat. No. 4,441,124 to Heebner et al., U.S. Pat. No. 4,614,427 to Koizumi et al., U.S. Pat. No. 4,889,998 to Hayano et al., and U.S. Pat. No. 5,317,380 to Allemand, all of which are incorporated by reference as if fully set forth herein.
In typical practice, however, the electrical signals are digitized to form an image of the scattered light. Further, the illumination area may be configured to be less than the specimen area, and then for fall coverage of the specimen, the specimen must move relative to the illumination source. Similarly, the detector may be configured to capture scattered light from an area less than the specimen area, and then for full coverage of the specimen, the specimen must move relative to the detection system. Typically, the illumination areas and detection areas are approximately equivalent in shape and size. There are three arrangements commonly used in inspection systems to collect images of whole specimens. An area well less than the dimensions of the specimen or wafer may be illuminated and imaged. By moving the specimen relative to the illuminator and detector in two dimensions, small area images may be collected, and a composite of the whole specimen may be formed by “stitching” or combining these small area images together. Alternatively, and as described by Addiego, an area with one dimension as larger or larger than the dimensions of the specimen and the other dimension well less than the dimensions of the specimen may be illuminated and imaged. By moving the specimen relative to the illuminator and detector in the direction substantially perpendicular to the long dimension of the illuminated area, a line scan image may be collected and then compiled into an image of the whole specimen. A third method illuminates the full specimen surface and collects a single image of the entire surface area of the specimen. In this case, the specimen may not need to move relative to the illumination and detection systems.
All three methods have been employed in prior art inspection systems. However, the prior art also is comprised of illumination and detection systems that use conventional optical systems composed of conventional lenses and detection sensors. For example, as shown in FIG. 1, a conventional optical system for a line scanning inspection system may include a conventional light source such as linear light source 10. In addition, a conventional lens may include lens 12 which may be configured to collect a line of scattered light rays 14 along a full length of a field of interest such as diameter 16 of specimen or wafer 18. Such a lens may be configured to direct the collected light rays 20 toward a camera that may include array 22 of charge-coupled device (“CCD”) sensors. Often, conventional optical systems can be extremely expensive, may include very large optical components, and may have very large optical paths. Such disadvantages become increasingly important as the dimensions of the specimens increase. For example, the linear light sources in a line scanning system typically should have a length that is approximately as long as a diameter of the wafer specimen. Currently available macro-page defect line scanning systems employ linear light sources with demonstrated acceptable uniformity for specimens up to 200 mm wide. However, as the diameter of substrates increases to 300 mm and beyond, the length of such linear light sources must also increase proportionally to the increase in the diameter of the substrates. Such conventional light sources, however, may not have acceptable uniformities over such a larger length.
To ensure that defects can be discerned from effects that arise from illuminating the surface structures of the semiconductor devices being formed, the imaging optics must also be uniform across the specimen dimensions of interest. Specifically, the optimal imaging system should collect light at angles that are equivalent across the full surface area of interest. However, for the case of large specimen objects such as a 200 mm wafer, practical configurations of image collection optics that collect light with substantially the same collection angles across an entire surface often result in optical path dimensions that are quite large and components that are quite costly.
Using conventional optics, imaging all points equivalently may be addressed in a number of ways. For example, an imaging lens may be positioned very far away from a specimen surface. Placing the imaging lens very far away from the surface, however, may only minimize variations across the surface of interest and may result in poor light collection capabilities. Such an approach has several disadvantages such as a long optical path and difficulties associated with collecting sufficient light such that an acceptable throughput may be achieved. A long optical path may be addressed by using a number of mirrors that may fold an optical path with little loss or distortion of signal. Such an optical system, however, may dramatically increase the complexity of fabrication and alignment of the system.
Alternatively, as shown in FIG. 2, large diameter optical components comparable in size to the surface size of interest such as lens 24 or mirrors may be included in the optical assembly and may be positioned very close to specimen 26. For example, lens 24 may be spaced above the surface of the specimen 26 by height 28 typically of the order of tens of millimeters. Lens 24 may also be configured to collect a line of scattered light rays 30 across an entire field of interest such as diameter 32 of specimen 26. Such optical components may be arranged to collect light normal to a wafer surface of to result in a substantially telecentric optical system as shown by parallel scattered light rays 30. (A telecentric configuration is advantageous because it satisfies the requirement for uniformity in the imaging optics.) Establishing telecentricity using such a large diameter optical component, however, results in long optical path length 34 between lens and sensor array 36 typically of the order of hundreds of millimeters. Such large diameter optical components may be very expensive because the lenses need to be as large as the specimen. As shown in FIG. 2, a diameter of lens 24 must be greater than or equal to approximately a diameter of specimen 26 which may be approximately 300 mm. The cost of such a lens scales as approximately d4, where d is the diameter of the specimen or wafer being imaged.
An example of a method for illuminating the entire surface area of a wafer is illustrated by Komatsu et al. in “Automatic Macro Inspection System,” SPIE, Spring, 2000, which is incorporated by reference as if fully set forth herein. As shown in FIGS. 3A and 3B, such an inspection system includes large optical components such as mirror 38 which has a diameter approximately equal to a diameter of wafer 40. Mirror 38 is shown to be configured to direct and “fold” the light returned from a wafer surface 40 to sensor 42 which may be a CCD camera. For example, as shown in FIG. 3A, the wafer may be positioned with respect to the optical components such that scattered light may be directed by mirror 38 to sensor 42. Alternatively, as shown in FIG. 3B, the wafer may be positioned at tilting angle 44 with respect to the optical components such that diffracted light is directed by mirror 38 to sensor 42.
In addition, as shown in FIG. 3A, the prior art inspection system may also include long optical path lengths to provide uniform illumination from single point light source 46. A long optical path length of hundreds of millimeters is typically required to achieve telecentricity or near-telecentricity. Alternatively, as shown in FIG. 3B, such an inspection system may include diffuser 48 configured to create “full sky” illumination of an entire wafer surface area 40. Large optical components such as mirror 38 and diffuser 48, however, can be very expensive. Imaging a wafer can require a large field lens having a diameter approximately equal to the diameter of a wafer specimen.
Note that because conventional inspection systems typically have optical assemblies in which the illumination system and the detection system are separately mounted within the inspection system, often extensive calibration and preventive maintenance work are required to ensure that the systems are performing adequately.
As indicated previously, the semiconductor industry is increasingly moving towards fabrication of semiconductor devices on 300 mm semiconductor substrates to increase manufacturing yield and throughput. It is anticipated that processing of 300 mm semiconductor substrates will be fully automated or at least may require substantial mechanical handling of the substrates to minimize overall semiconductor device fabrication costs. For example, semiconductor fabrication facilities will likely include tracks configured to transport semiconductor substrates into and out of various fabrication tools. In this manner, clean room space for a tool is more efficiently utilized and costs of maintaining the clean room space can thus be minimized. Increased automation is desired to reduce human handling of the semiconductor substrates and the associated additional risks of contamination. In an automated fabrication line, continuous wafer flow is critical, and typically, flow rates are paced by the slowest module in the line. Typically, process tools have priority over inspection tools, and hence, the wafer flow in inspection tools must not impede overall wafer flow in the line. The wafer flow, or throughput, through an inspection tool must then be at least comparable to that of the process tools preceding it. Current state of the art lithography processing tools operate at >100 wafers per hour, and versions supporting 300 mm sized substrates are anticipated to run as high as 150 wafers per hour or more. All these adjustments being adopted for semiconductor fabrication of 300 mm wafers set changes or new requirements for the design of inspection tools. Inspection tools that have been developed for inspection of 200 mm semiconductor substrates may not be directly applicable in the semiconductor fabrication lines using 300 mm wafers, and thus may need to be completely, or at least significantly, redesigned to accommodate the new size and fabrication methodologies being introduced using 300 mm wafers.
The simplest approaches to designing inspection systems for inspection of 300 mm semiconductor substrates merely scale the technologies developed for inspection systems designed for 200 mm semiconductor substrates. However, several significant difficulties may arise in scaling current technologies. For example, maintaining low fabrication costs for imaging lenses that are larger and in proportion to the increased diameter of substrates and that maintain minimum distortion may be extremely difficult. Cost of optical elements increase rapidly with increases in a diameter (approximately on the order of d4). An additional difficulty is ensuring equivalent or improved illumination uniformities for larger diameter substrates.
To support full automation to optimize processing flow and floor space usage, and to minimize errors introduced by human handling, thus minimizing overall cost, integrated process lines are anticipated for the fabrication of 300 mm-sized substrates. In this case, the inspection tools become part of the overall fabrication process line. Specifically, wafers might be transported directly from a process module directly and automatically into an inspection module through a track or using some other wafer handling device, and when the wafer has been inspected, it is removed from the inspection module and moved directly to the next process module using a wafer handling device. Currently, semiconductor fabrication process lines for substrates ≦200 mm in size contain some process and inspection tools that are integrated, and some that are stand-alone. In the case of the stand-alone tools, for example, a user must transport specimens from one process tool to the inspection tool, and then remove them and place them into the next process tool. Because some tools were intended to operate as stand-alone tools, these may have vertical and lateral dimensions that make integration into a semiconductor fabrication process line impractical. An inspection tool having smaller profile, but maintaining the inspection capabilities of stand-alone tools, may therefore have advantages attractive to integrated process lines.
To ensure that an inspection tool's throughput at least meets the semiconductor fabrication process line wafer flow requirements, the tool architecture for image capture and processing must be well optimized for time. The throughput of an inspection tool is paced by the time to load and unload wafers in the inspection module, the time to capture an image, and the time to analyze the image. An optimized inspection tool architecture may place image analysis in parallel with one of the other two key time components. Of these two remaining key time components, the time to capture an image is of most interest for this invention. Specifically, and as discussed above, image capture is a function of the illumination system and detection system of the inspection tool. Further, the time to capture an image is the time required to collect a sufficient amount of light scattered from the specimen surface, so that further processing of the digitized signal or image that results from the conversion of the collected light can discern the defects of interest. This collection time is also known as an exposure time, and specifically, is a function of the total amount of light provided to the specimen surface by the illumination system, the amount of light directed by the detection system optics, and the collection efficiency of the detection sensors. If, for example, the illumination source is very dim, then the amount of time required to collect sufficient light for an image that can discern the defects of interest may be very long. In the case of scaling conventional illumination system optics and conventional detection system optics to accommodate larger specimen sizes such as 300 mm wafers, delivery of sufficient light to the specimen surface and delivery of sufficient light to the detection sensors may become increasingly difficult without increase in the output of the illumination source itself. Specifically, illumination using the same illumination source power and scaled optics may result in reducing the illumination per area by at worst the square of the ratio of specimen size differences, and at best as the ratio of the specimen size differences, depending on the size and shape of the illumination area. For example, in scaling a full specimen illumination configuration from 200 mm diameter to 300 mm diameter, the total illumination per area may be reduced by (100/150)2 or about 44%. For a line scan system, the reduction in illumination per area may be 200/300 or about 66%. In either of these cases, the exposure time may need to be increased to ensure that sufficient light is collected to provide an image that can discern the defects of interest. Increasing the exposure time results in decreasing the overall throughput. To compensate for the loss in illumination per area, the illumination source power may be increased. This may increase cost. Alternatively, the optical paths if conventional components are used may require re-design to increase delivery efficiencies. Increased costs and/or complexity may result.